body {
    background-color: #000;
    color: #fff;
}

.container {
    display: flex;
    align-items: stretch;
    justify-content: space-around;
}

.红绿灯 {
    width: 100px;
    height: 100px;
    border-radius: 50%;
}

.红灯 {
    background-color: rgb(255, 0, 0);
}

.黄灯 {
    background-color: rgb(255, 255, 0);
}

.绿灯 {
    background-color: rgb(0, 255, 0);
}